1. Field of the Invention
The invention relates in general to a pixel of display, and more particularly to an organic light emitting diode (OLED) display pixel.
2. Description of the Related Art
Referring to FIG. 1, a circuit diagram of a conventional organic light emitting diode (OLED) display pixel is shown. OLED pixel 100 comprises a first transistor Q1, a second transistor Q2, a third transistor Q3, a fourth transistor Q4, a capacitor C, and an organic light emitting diode. The current driving method of the OLED pixel 100 is disclosed below. The driving circuit (not shown in FIG. 1) provides various currents (a constant source current is exemplified here) according to various gray values, so the pixel 100 generates a corresponding voltage Vc across capacitor C according to the data current I of the constant source current output. That is to say, when the current that flows through the first transistor Q1 is almost equal to the data current I, the voltage Vgs1 between the gate and the source of the first transistor Q1 corresponding to the current flowing through the first transistor Q1 is stored at the capacitor C and becomes a capacitor cross-voltage Vc. Therefore, when the scanning signal S is disabled, the third transistor 03, the fourth transistor Q4 and the first transistor Q1 are all turned off. Given that the first transistor Q1 and the second transistor Q2 have the same characteristics, and that the capacitor cross-voltage Vc is almost maintained at the level of the voltage Vgs1, the voltage Vgs2 between the gate and the source of the second transistor Q2 would be substantially equal to the voltage Vgs1, meanwhile, the current flowing through the second transistor Q2 would be theoretically equal to the data current I.
In practice, shift in the threshold voltage would occur to the transistor after long duration of operation. The shift has much to do with the operation time of the transistor and the volume of the current flowing through. The turn-on duration of the first transistor Q1 is different from that of the second transistor Q2, because the second transistor Q2 is always turned on. The second transistor Q2 is turned on no matter the scanning signal S is enabled or disabled, while the first transistor Q1 is turned on only when the scanning signal S is enabled. Therefore, the shift in the threshold voltage of conduction for the first transistor Q1 is different from that for the second transistor Q2. The voltage Vgs2 between the gate and the source of the second transistor Q2 and the voltage Vgs1 between the gate and the source of the first transistor may both be equal to the capacitor cross-voltage Vc, but the shift in the threshold voltage for the first transistor Q1 is different from that for the second transistor Q2, resulting in different threshold voltages between the second transistor Q2 and the first transistor Q1. Consequently, the volume of the current generated by the second transistor Q2 would be different from the data current I, preventing the OLED luminance from achieving the corresponding luminance of the data current I.